The present invention relates generally to a scroll compressor and, more particularly, to the shape of spiral wrap elements of stationary and orbiting scrolls of the scroll compressor.
Generally available scroll compressors comprise a stationary scroll and an orbiting scroll, both of which have respective wrap elements each having an internal wall and an external wall opposite to each other.
In almost all of the scroll compressors, the internal and external walls are shaped so as to represent two involute curves, both of which are based on an identical involute base circle, but have different phases and, hence, the wall thickness thereof is made constant from an inner end to an outer end of each of the wrap elements. The compressing action is achieved by circling one of the wrap elements relative to the other under the condition in which the two wrap elements are circumferentially shifted 180.degree. one from the other with the side walls thereof held in partial contact with each other.
Alternatively, the internal wall of one of the two wrap elements can be shaped to represent a curve developing from the center of a base circle so as to extend spirally outwardly therefrom, while the external wall of said one of the two wrap elements is shaped based on the shape of the internal wall. In this case, the other wrap element is shaped with envelope curves obtained as a result of circular translation of said one wrap element relative thereto. The scroll compressors of this type have the advantage of being capable of enlarging the volume of entrapment, if the cylinder bore is identical, by making the factor of development (a) large, compared with those of the aforementioned type having the wrap elements so shaped as to represent the involute curves. If the intake volume is identical, the outer diameter of a shell can be reduced, contributing to the compactness or light-weight of the whole compressor.
However, if the wrap elements are shaped in this way, said one wrap element becomes thin at an inner end portion thereof, while the other wrap element inevitably has a sharp edge at an inner end portion thereof. Such wrap elements are shown in, for example, FIG. 1, requiring a specific change in shape to increase the strength thereof.
Several patent documents including U.S. Pat. No. 4,547,137 disclose an improved shape of the inner end portions of the wrap elements. According to these documents, the two wrap elements are shaped symmetrically by merely connecting involute curves with arcuate or straight lines. If the two wrap elements are of asymmetrical shapes, they cannot be shaped in a simple manner but must be shaped in consideration of the circular translation of one of them relative to the other. Furthermore, if the inner end walls of the wrap elements are unnecessarily made thick for the purpose of merely increasing the strength thereof, light-weight compressors cannot be expected. Also, enlargement in volume of entrapment of a final chamber delimited by the inner end walls of the two wrap elements results in a reduction in compression ratio or a re-expansion loss, adversely affecting the performance of the compressors.